(1) Field of the invention
The present invention relates to a system and method for controlling fuel combustion according to an octane number of fuel used for an internal combustion engine mounted in a vehicle body.
(2) Background of the art
When a spark ignited engine is driven using a certain kind of fuel, with its compression ratio increased and/or the ignition timing advanced, engine knocking would occur. The engine knocking would cause overheating and/or reduction in performance of the engine, and, in a worst case, would cause damage to the engine structure.
An octane number of the fuel used for the engine represents an anti-knock characteristic of the fuel to prevent such a reduction in engine performance as described above.
The octane numbers of gasoline fuels commercially available on the Japanese market usually are indicated as follows:
Regular (Low-octane) gasoline--about 91 (octane); and
Super (High-octane) gasoline--about 98 (octane).
It is necessary to increase combustion speed as high as possible and so as not to generate engine knock in terms of the thermal efficiency of the engine. The high-octane gasoline is advantageous due to its high octane number when used as the fuel for an engine in which advancement of ignition timing is carried out to control the fuel combustion to avoid engine knocking.
If the regular (low-octane) gasoline is erroneously or inevitably used due to the lack of availability of the high-octane gasoline for the fuel in an engine designed to use the high-octane gasoline as the fuel, a sufficient engine output corresponding to that for the case of using the high-octane gasoline could not be obtained or engine knocking would frequently occur. Consequently, engine driveability worsens.
A Japanese Patent Application First Publication (Non-examined) No. Sho60-212673 published on Oct. 24, 1985 exemplifies an engine ignition timing controlling system in which two combustion controlling maps for the regular gasoline and for the high-octane gasoline are prepared and the two maps are automatically switched together to use for the fuel combustion control in each combustion chamber of the engine according to the octane number of the fuel used in order to solve the above-described problem.
In the ignition timing controlling system disclosed in the above-identified Japanese Patent Application First Publication, the ignition timing control is carried out using one of the maps for the regular gasoline prepared for such a low-octane gasoline after the engine is started. Thereafter, the ignition timing is advanced at a constant rate of advance angles when the engine operating condition falls in a predetermined determination area. When the ignition timing angle is advanced and exceeds a thresold value without the occurrence of knocking, the system determines that the fuel has the high octane number and executes another ignition control using the other map prepared for the high-octane gasoline fuel. In addition, when the knocking occurs during use of the other map for the high-octane gasoline, the ignition timing is retarded. At this time when a retardation amount (retard angle of the ignition timing) exceeds a maximum retardation amount, the map used for the ignition timing control is switched to the map prepared for the low-octane gasoline.
However, since in the above-described ignition timing controlling system disclosed in the above-identified Japanese Publication a difference between each basic control value for the high-octane gasoline and for the low-octane gasoline is large if the system erroneously determines that the fuel used in the engine designed to meet the high-octane gasoline fuel is the low-octane gasoline due to weather conditions or other reasons although the actual fuel is the high-octane gasoline, the ignition timing is excessively retarded (exceeding the maximum retardation amount) with respect to a natural fuel characteristic. Therefore, the map use is not permanently returned to the map prepared for the high-octane gasoline when the engine has been started.
Consequently, the engine output corresponding to that in the case of using the high-octane gasoline cannot be obtained. In addition, the engine performance is further deteriorated as compared with the case using the low-octane gasoline and, in the worst case, the engine would be damaged due to the increase in exhaust gas temperature.